Precision Measurement of Net-Proton-Number Fluctuations in Au+Au Collisions at RHIC

We report precision measurements on cumulants (Cn) and factorial cumulants (κn) of (net) proton number distributions up to fourth order in Au+Au collisions over center-of-mass energies sNN=7.7-27 GeV from phase II of the Beam Energy Scan program at RHIC. (Anti)protons are selected at midrapidity (|y|<0.5) within a transverse momentum range of 0.4<pT<2.0 GeV/c. Relative to various noncritical-point model calculations and peripheral collision 70%-80% data, the net proton C4/C2 measurement in 0%-5% collisions shows a minimum around 19.6 GeV for significance of deviation at ∼2-5σ. A minimum in C4/C2 with respect to a noncritical baseline is expected to be a characteristic feature of the signature associated with a critical point in the QCD phase diagram. In addition, deviations from noncritical baselines around the same collision energy region are also seen in proton factorial cumulant ratios, especially in κ2/κ1 and κ3/κ1. Dynamical model calculations including a critical point are called for in order to understand these precision measurements.